Indigo is a well-known vat dye used for dyeing cellulose-containing textile material, in particular cotton warp yarns for blue denim articles.
Indigo is water-insoluble, and to apply it it must first be reduced (vatted) into the water-soluble leuco form, which, having gone onto the material to be dyed, is oxidized back to the pigment.
In the customary dyeing processes, indigo is vatted in an alkali medium in a vessel upstream of the dyebath using inorganic reducing agents such as sodium dithionite and thiourea dioxide or else organic reducing agents such as hydroxyacetone. Additional machine-dependent portions of reducing agent are consumed during the dyeing process, since part of the leuco indigo is oxidized by air contact in the air passages and at the dyebath surface and has to be revatted, which is why the dyebath too has reducing agent added to it (up to 70% of the total required).
The disadvantage of vatting indigo with the reducing agents mentioned is the high concentration in the dyehouse waste water of sulfate (from about 3500 to 5000 mg/l, measured in the waste water of an indigo dyehouse) in the case of sodium dithionite or of oxygen-consuming substances (COD values about 8000 mg/l, measured in the waste water from an indigo dyehouse) in the case of hydroxyacetone. In addition to sodium dithionite, for example, the reduction requires a high level of alkali.
Also known are solid and pasty formulations of leuco indigo which in addition to alkali contain in particular sugar-derived polyhydroxy compounds, in particular molasses, as stabilizers intended to prevent oxidation to form indigo. When these formulations are used for dyeing, the sulfate content of the dyehouse waste water can indeed be effectively lowered, but the waste water becomes highly polluted with dissolved organic hydroxy compounds, measured as TOC (total organic carbon), COD (chemical oxygen demand) or BOD (biological oxygen demand); for example, a 60% by weight leuco indigo formulation with 25% by weight of molasses results in a COD of about 8000 mg/l. In addition, molasses diminishes the reducing effect of the reducing agent added to the dyebath (particularly sodium dithionite and thiourea dioxide) and interferes with the setting of the optimum dyeing pH, resulting in unsatisfactory results, inter alia a watery appearance and insufficient brilliance of the dyeing.
Finally, WO 90/15182 discloses a dyeing process wherein indigo is added to the dyebath in oxidized form and electrochemically reduced in the dyebath using mediators. Mediators are reversible redox systems which reduce the dye, being oxidized in the process, are rereduced at the cathode and are available again for dye reduction. However, reduction of the total amount of indigo necessary for dyeing requires enormous amounts of charge and large electrode surfaces. In addition, conducting salt has to be added to the dyeing solution in major quantities (about 1.5 g/l of NaOH, 30 g/l of Na.sub.2 SO.sub.4) to ensure adequate electrical conductivity and hence to minimize the resistance losses and also the electrode surface area. This in turn leads to undesirably high levels of sulfate in the dyehouse waste water.